Various devices are known for depositing blood cells on microscope slides. Among these are the devices described in U.S. Pat. Nos. 3,705,048 and 3,906,890. These devices rotate a slide about an axis perpendicular to the surface of the slide itself such that blood deposited on the slide is driven by centrifugal force across the surface of the slide, thereby widely distributing the blood cells of the sample. Such devices provide a relatively rapid, uniform technique for effecting blood counts and the like.
In other applications it is desirable to obtain a greater concentration of cells on the slide and to somewhat flatten the cells so that their structure may be more readily ascertained. One device for this purpose is available and sold under the name "Cytospin" by Shandon-Elliott. Such device utilizes a bowl-type centrifuge rotor whose outer periphery defines a vertical wall adapted to receive microscope slides. Chambers for holding samples to be sedimented are positioned radially against the slides with a piece of filter paper between each chamber and its slide. A hole in the filter paper is positioned over an outlet orifice of the chamber such that cells in a fluid introduced into the chamber, when centrifuged, are driven against the slide. The filter paper serves the function of withdrawing excess fluid from the surface of the slide such that the sedimented cells can remain in position following centrifugation.
Unfortunately, the filter paper can have a deleterious effect. It tends to absorb the fluid so rapidly that the cells are literally "sucked" or carried with the fluid into the peripheral edges of the filter paper surrounding the outlet orifice with relatively few cells having sufficient time to pellet or sediment against the slide itself. Also, the cells remaining on the slide tend to become dired since the filter paper absorbs most of the fluid. Generally, it is more desirable to sediment the cells while they are wet since they tend to remain rounded unless the applied centrifugal force exceeds the osmotic pressure of the cell. Another problem encountered with the use of filter paper is that the sample volume that can be used is relatively small. For this reason, usually only the cells of a single sample can be deposited on a given slide. The buildup of cells by the sedimentation of multiple sample on a single slide is difficult if not impossible to achieve.
A device similar to the Shandon-Elliott unit is described in an article entitled "A Device for Preparing Cell Spreads" by C. F. Dori et al., Immunology, 1965, 9, 403. Dori et al. note if the centrifugal force on the slides (and filter paper) is insufficient, a majority of the cells are pulled into the filter paper. Conversely, they note, too great a force or pressure prevents the preparations from drying in a short period of time (15 minutes). It is thus apparent that the devices of the prior art for preparing particle spreads on slides are somewhat critical and not entirely satisfactory.
A centrifuge rotor alleviating many of these difficulties is described and claimed in a copending application, Ser. No. 015,911, filed Feb. 28, 1979, by J. W. Boeckel et al. and assigned to the same assignee as the subject invention. In this rotor, conduit means communicate with each chamber for withdrawing excess fluid and are coupled to the rotor hub and thence through a fluid rotating seal to a suitable vacuum source. While quite satisfactory in general, this prior art device does encounter two disadvantages. One is that each vacuum line must be individually sealed off if it is not in use or else the vacuum may be decreased to a point which is undesirable. Secondly, since the fluid in the chambers are sucked generally upward, fluid sometimes becomes entrapped in cavities within the exhaust lines or conduits such that when a new slide is positioned for a run, some of the fluid from the preceding slide may fall back and contaminate the new slide.